Operating Systems

Start Lecture #22

Remark: Lab 4 is available and is due in 2 weeks (23 April 2009).

3.7 Segmentation

Up to now, the virtual address space has been contiguous. In segmentation the virtual address space is divided into a number of variable-size segments. One can view the designs we have studied so far as having just one segment, the entire process.

• Among other issues this makes memory management difficult when there are more that two dynamically growing regions.
• With two regions you start them on opposite sides of the virtual space as we did before.
• Better is to have many virtual address spaces each starting at zero.
• This split up is user visible. So a segment is a logical split up of the address space. Unlike with (user-invisible) paging, segment boundaries occur at logical point, e.g., at the end of a procedure.
• Imagine a system with several large, dynamically-growing, data structures. The same problem we mentioned for the OS when there are more than two growing regions, occurs as well for user programs. The user (or some user-mode tool) must decide how much virtual space to leave between the different tables. With segmentation
• Eases flexible protection and sharing: One places in a single segment a unit that is logically shared. This would be the natural method to implement shared libraries.
• When shared libraries are implemented on paging systems, the design essentially mimics segmentation by treating a collection of pages as a segment. This is more complicated since one must ensure that the end of the unit to be shared occurs on a page boundary (this is done by padding).
• Without segmentation (equivalently said with just one segment) all procedures are packed together so, if one changes in size, all the virtual addresses following this procedure are changed and the program must be re-linked. With each procedure in a separate segment this relinking would be limited to the symbols defined or used in the modified procedure.

Homework: Explain the difference between internal fragmentation and external fragmentation. Which one occurs in paging systems? Which one occurs in systems using pure segmentation?

** Two Segments

Late PDP-10s and TOPS-10

• Each process has one shared text segment, that can also contain shared (normally read only) data. As the name indicates, all process running the same executable share the same text segment.
• The process also contains one (private) writable data segment.
• Permission bits define for each segment.

** Three Segments

Traditional (early) Unix had three segments as shown on the right.

1. Shared text marked execute only.
2. Data segment (global and static variables).
3. Stack segment (automatic variables).

Since the text doesn't grow, this was sometimes treated as 2 segments by combining text and data into one segment. But then the text could not be shared.

** General (Not Necessarily Demand) Segmentation

Segmentation is a user-visible division of a process into multiple variable-size segments, whose sizes change dynamically during execution. It enables fine-grained sharing and protection. For example, one can share the text segment as done in early unix.

With segmentation, the virtual address has two components: the segment number and the offset in the segment.

Segmentation does not mandate how the program is stored in memory.

• One possibility is that the entire program must be in memory in order to run it. Use whole process swapping. Early versions of Unix did this.
• Can also implement demand segmentation (see below).
• More recently, segmentation is combined with demand paging (done below).

Any segmentation implementation requires a segment table with one entry for each segment.

• A segment table is similar to a page table.
• Entries are called STEs, Segment Table Entries.
• Each STE contains the base address of the segment and the limit value (the size of the segment).
• Why is there no limit value in a page table?
• Answer: All pages are the same size so the limit is obvious.

The address translation for segmentation is
    (seg#, offset) --> if (offset<limit) base+offset else error.

3.7.1 Implementation of Pure Segmentation

Pure Segmentation means segmentation without paging.

Segmentation, like whole program swapping, exhibits external fragmentation (sometimes called checkerboarding). (See the treatment of OS/MVT for a review of external fragmentation and whole program swapping). Since segments are smaller than programs (several segments make up one program), the external fragmentation is not as bad as with whole program swapping. But it is still a serious problem.

As with whole program swapping, compaction can be employed.

** Demand Segmentation

Same idea as demand paging, but applied to segments.

• If a segment is loaded, base and limit are stored in the STE and the valid bit is set in the STE.
• The STE is accessed for each memory reference (not really, there is probably a TLB).
• If the segment is not loaded, the valid bit is unset. The base and limit as well as the disk address of the segment is stored in the an OS table.
• A reference to a non-loaded segment generate a segment fault (analogous to page fault).
• To load a segment, we must solve both the placement question and the replacement question (for demand paging, there is no placement question).
• Pure segmentation was once implemented by Burroughs in the B5500. I believe the implementation was in fact demand segmentation but I am not sure. Demand segmentation is not used in modern systems.

The table on the right compares demand paging with demand segmentation. The portion above the double line is from Tanenbaum.

** 3.7.2 and 3.7.3 Segmentation With (Demand) Paging

These two sections of the book cover segmentation combined with demand paging in two different systems. Section 3.7.2 covers the historic Multics system of the 1960s (it was coming up at MIT when I was an undergraduate there). Multics was complicated and revolutionary. Indeed, Thompson and Richie developed (and named) Unix partially in rebellion to the complexity of Multics. Multics is no longer used.

Section 3.7.3 covers the Intel Pentium hardware, which offers a segmentation+demand-paging scheme that is not used by any of the current operating systems (OS/2 used it in the past). The Pentium design permits one to convert the system into a pure damand-paging scheme and that is the common usage today.

I will present the material in the following order.

1. Describe segmentation+paging (not demand paging) generically, i.e. not tied to any specific hardware or software.
2. Note the possibility of using demand paging (again generically).
3. Give some details of the Multics implementation.
4. Give some details of the Pentium hardware, especially how it can emulate straight demand paging.

** Segmentation With (non-demand) Paging

One can combine segmentation and paging to get advantages of both at a cost in complexity. In particular, user-visible, variable-size segments are the most appropriate units for protection and sharing; the addition of (non-demand) paging eliminates the placement question and external fragmentation (at the smaller average cost of 1/2-page internal fragmentation per segment).

The basic idea is to employ (non-demand) paging on each segment. A segmentation plus paging scheme has the following properties.

• A virtual address becomes a triple: (seg#, page#, offset).
  
  
• Each segment table entry (STE) points to the page table for that segment. Compare this with a multilevel page table.
  
  
• The physical size of each segment is a multiple of the page size (since the segment consists of pages). The logical size is not; instead we keep the exact size in the STE (limit value) and terminate the process if it references beyond the limit. In this case the last page of each segment is partially wasted (internal fragmentation).
  
  
• The page# field in the address gives the entry in the chosen page table and the offset gives the offset in the page.
  
  
• From the limit field, one can easily compute the size of the segment in pages (which equals the size of the corresponding page table in PTEs).
  
  
• A straightforward implementation of segmentation with paging would requires 3 memory references (STE, PTE, referenced word) so a TLB is crucial.
  
  
• Some books carelessly say that segments are of fixed size. This is wrong. They are of variable size with a fixed maximum and with the requirement that the physical size of a segment is a multiple of the page size.
  
  
• Keep protection and sharing information on segments. This works well for a number of reasons.
  1. A segment is variable size.
  2. Segments and their boundaries are user-visible
  3. Segments are shared by sharing their page tables. This eliminates the problem mentioned above with shared pages.
  
  
• Since we have paging, there is no placement question and no external fragmentation.
  
  
• The problems are the complexity and the resulting 3 memory references for each user memory reference. The complexity is real. The three memory references would be fatal were it not for TLBs, which considerably ameliorate the problem. TLBs have high hit rates and for a TLB hit there is essentially no penalty.

Although it is possible to combine segmentation with non-demand paging, I do not know of any system that did this.

Homework: 36.

Homework: Consider a 32-bit address machine using paging with 8KB pages and 4 byte PTEs. How many bits are used for the offset and what is the size of the largest page table? Repeat the question for 128KB pages. So far this question has been asked before. Repeat both parts assuming the system also has segmentation with at most 128 segments.

Homework: Consider a system with 36-bit addresses that employs both segmentation and paging. Assume each PTE and STE is 4-bytes in size

1. Assume the system has a page size of 8K and each process can have up to 256 segments. How large in bytes is the largest possible page table? How large in pages is the largest possible segment?
2. Assume the system has a page size of 4K and each segment can have up to 1024 pages. What is the maximum number of segments a process can have? How large in bytes is the largest possible segment table? How large in bytes is the largest possible process.
3. Assume the largest possible segment table is 2¹³ bytes and the largest possible page table is 2¹⁶ bytes. How large is a page? How large in bytes is the largest possible segment?

** Segmentation With Demand Paging

There is very little to say. The previous section employed (non-demand) paging on each segment. For the present scheme, we employ demand paging on each segment, that is we perform fetch-on-demand for the pages of each segment.

The Multics Scheme

Multics was the first system to employ segmentation plus demand paging. The implementation was as described above with just a few wrinkles, some of which we discuss now together with some of the parameter values.

• The Multics hardware (GE-645) was word addressable, with 36-bit words (the 645 predates bytes).
• Each virtual address was 34-bits in length and was divided into three parts as mentioned above. The seg# field was the high-order 18 bits; the page# field was the next 6 bits; and the offset was the low-order 10 bits.
• The actual implementation was more complicated and the full 34-bit virtual address was not present in one place in an instruction.
• Thus the system supported up to 2¹⁸=256K segments, each of size up to 2⁶=64 pages. Each page is of size 2¹⁰ (36-bit) words.
• Since the segment table can have 256K STEs (called descriptors), the table itself can be large and was itself demand-paged.
• Multics permits some segments to be demand-paged while other segments are not paged; a bit in each STE distinguishes the two cases.

The Pentium Scheme

The Pentium design implements a trifecta: Depending on the setting of a various control bits the Pentium scheme can be pure demand-paging, pure segmentation, or segmentation with demand-paging.

The Pentium supports 2¹⁴=16K segments, each of size up to 2³² bytes.

• This would seem to require a 14+32=46 bit virtual address, but that is not how the Pentium works. The segment number is not part of the virtual address found in normal instructions.
• Instead separate instructions are used to specify which are the currently active code segment and data segment (and other less important segments). Technically, the CS register is loaded with the selector of the active code segment and the DS register is loaded with the selector of the active data register.
• When the selectors are loaded, the base and limit values are obtained from the corresponding STEs (called descriptors).
• There are actually two flavors of segments, some are private to the process; others are system segments (including the OS itself), which are addressable (but not necessarily accessible) by all processes.

Once the 32-bit segment base and the segment limit are determined, the 32-bit address from the instruction itself is compared with the limit and, if valid, is added to the base and the sum is called the 32-bit linear address. Now we have three possibilities depending on whether the system is running in pure segmentation, pure demand-paging, or segmentation plus demand-paging mode.

1. In pure segmentation mode the linear address is treated as the physical address and memory is accessed.

2. In segmentation plus demand-paging mode, the linear address is broken into three parts since the system implements 2-level-paging. That is, the high-order 10 bits are used to index into the 1st-level page table (called the page directory). The directory entry found points to a 2nd-level page table and the next 10 bits index that table (called the page table). The PTE referenced points to the frame containing the desired page and the lowest 12 bits of the linear address (the offset) finally point to the referenced word. If either the 2nd-level page table or the desired page are not resident, a page fault occurs and the page is made resident using the standard demand paging model.

3. In pure demand-paging mode all the segment bases are zero and the limits are set to the maximum. Thus the 32-bit address in the instruction become the linear address without change (i.e., the segmentation part is effectively) disabled. Then the (2-level) demand paging procedure just described is applied.

   Current operating systems for the Pentium use this last mode.

3.9 Summary

Read

Some Last Words on Memory Management

We have studied the following concepts.

• Segmentation / Paging / Demand Loading (fetch-on-demand).
  • Each is a yes or no alternative.
  • This gives 8 possibilities.
• Placement and Replacement.
• Internal and External Fragmentation.
• Page Size and locality of reference.
• Multiprogramming level and medium term scheduling.

Chapter 4 File Systems

There are three basic requirements for file systems.

1. Size: Store very large amounts of data.
2. Persistence: Data survives the creating process.
3. Concurrent Access: Multiple processes can access the data concurrently.

High level solution: Store data in files that together form a file system.

4.1 Files

4.1.1 File Naming

Very important. A major function of the file system is to supply uniform naming. As with files themselves, important characteristics of the file name space are that it is persistent and concurrently accessible.

Unix-like operating systems extend the file name space to encompass devices as well

Does each file have a unique name?
Answer: Often no. We will discuss this below when we study links.

File Name Extensions

The extensions are suffixes attached to the file names and are intended to in some why describe the high-level structure of the file's contents.

For example, consider the .html extension in class-notes.html, the name of the file we are viewing. Depending on the system and application, these extensions can have little or great significance. The extensions can be

1. Conventions just for humans. For example letter.teq (my convention) signifies to me that this letter is written using the troff text-formatting language and employs the tbl preprocessor to handle tables and the eqn preprocessor to handle mathematical equations. Neither linux, troff, tbl, nor equ place any significance in the .teq extension.
   
   
2. Conventions giving default behavior for some programs.
   • The emacs editor by default edits .html files in html mode. However, emacs can edit such files in any mode and can edit any file in html mode. It just needs to be told to do so during the editing session.
   • The firefox browser assumes that an .html extension signifies that the file is written in the html markup language. However, having <html> ... </html> inside the file works as well.
   • The gzip file compressor/decompressor appends the .gz extension to files it compresses, but accepts a --suffix flag to specify another extension.
   
   
3. Default behaviors for the operating system or window manager or desktop environment.
   • Click on .xls file in windows and excel is started.
   • Click on .xls file in nautilus under linux and openoffice is started.
   
   
4. Required for certain programs.
   • The gnu C compiler (and probably other C compilers) requires C programs be have the .c (or .h) extension, and requires assembler programs to have the .s extension.
   
   
5. Required by the operating system
   • MS-DOS treats .com files specially.

Case Sensitive?

Should file names be case sensitive. For example, do x.y, X.Y, x.Y all name the same file? There is no clear answer.

• Unix-like systems employ case sensitive file names so the three names given above are distinct.
• Windows systems employ case insensitive file names to the three names given above are equivalent.
• Mathematicians (and others) often "consider an element x of a set X" so use case sensitive naming.
• Normal English (and other natural language) usage often employs case insensitivity (e.g. capitalizing a word at the beginning of a sentence does not change the word).